[InstCombine] Set MadeIRChange in replaceInstUsesWith.

[InstCombine] Set MadeIRChange in replaceInstUsesWith.

Some utilities used by InstCombine, like SimplifyLibCalls, may add new
instructions and replace the uses of a call, but return nullptr because
the inserted call produces multiple results.

Previously, the replaced library calls would get removed by
InstCombine's deleter, but after
292077072ec1279d89d21873fe900061e55ef936 this may not happen, if the
willreturn attribute is missing.

As a work-around, update replaceInstUsesWith to set MadeIRChange, if it
replaces any uses. This catches the cases where it is used as replacer
by utilities used by InstCombine and seems useful in general; updating
uses will modify the IR.

This fixes an expensive-check failure when replacing
@__sinpif/@__cospifi with @__sincospif_sret.